1. Field of the Invention
The present invention relates to a charged-particle beam optical apparatus for imaging a first mask including a plurality of apertures on a specimen to be irradiated, the mask being uniformly illuminated by a beam through a plurality of condenser lenses, and which includes means for adjusting the position of the mask relative to the specimen, a selected area of the specimen having an adjustment marking disposed thereon which is illuminated by a ray of charged particles of the beam passing through a test opening provided in the mask, and means for detecting radiation emanating from the specimen.
2. Description of the Prior Art
Charged-particle beam optical apparatus of the foregoing type are known in the art. See, for example, the paper by Heritage, J. Vac. Sci. Technol., Vol. 12 (1975), pages 1135 et seq. Such apparatus are particularly useful for generating patterns on semiconductor wafers in manufacturing integrated circuits.
In known charged-particle beam optical apparatus, excitation of the condenser lens system is increased according to the relative adjustment of the mask and the specimen so that assuming the lenses are ideal, the source of the charged-particle beam is imaged as a point on the plane of the mask and thereby also on the specimen plane as a point by the projection lens system which follows. A deflection system is located above the mask so that the point probe formed by the beam scans the test opening in the plane of the mask. This generates an image of the test opening in the plane of the specimen. A detector which registers secondary electrons released at the semiconductor wafer is located in the vicinity of the specimen and generates a detector signal which is transmitted to a monitor. An image containing the adjustment marking on the specimen, as well as the image of the test opening of the mask, then appears on the screen of the monitor. In order to adjust the images on the monitor screen, the mask or the specimen is shifted until the image of the test opening and that of the adjustment marking coincide. After this adjustment, the excitation of the condenser lens system must be changed so that the mask is integrally illuminated and its entire area is imaged on the specimen. It is difficult, however, to return the condenser lens excitation in an exact manner if the condenser lens which is to be readjusted is provided with an iron shell member, as is usually the case. Also, since the condenser lens system does not consist of ideal lenses, but in contrast causes imaging errors, the image of the beam source in the plane of the mask is not a point. As a result, the test opening in the mask can be imaged in the plane of the specimen only with unavoidable poor definition. The accuracy of the adjustment is, accordingly, limited.